The field of the invention is the formation of structures comprising a thin layer made of semiconductor material on a support substrate, by transferring the thin layer from a donor substrate to the support substrate. These structures are usually obtained by the use of a transfer process including steps to:
create a weakened zone within the thickness of a donor substrate;
bring the donor substrate into intimate contact with a support substrate;
detach the donor substrate at the weakened zone, to transfer part of the donor substrate onto the support substrate;
treat the part of the donor substrate transferred onto the support substrate to form the thin layer, with the treatment consisting of a sequence of finishing operations.
The invention more particularly relates to the formation of structures with a particularly thin layer, in other words the thickness of the thin layer is typically very thin, from about 100 to less than 1000 Angstroms, and in particular less than 500 Angstroms.
The well known SMART-CUT® processes are examples of transfer processes for making structures of this type and these correspond to a preferred embodiment of the invention. The structures formed by such processes are of the Semiconductor On Insulator (SeOI) type. Remember that in this case, an oxide layer is inserted between the support substrate and the thin layer. Structures obtained by such processes are used for applications in the microelectronics, optics and/or optronics fields.
After the donor substrate has been detached at the weakened zone, in order to transfer a part of the donor substrate onto the support substrate, it is known that that part must be treated to create the thin layer with the desired configuration and properties. Such a treatment is particularly designed to:
thin the part of the donor substrate transferred onto the support substrate, to bring it to the target thickness required for the thin layer;
reduce the surface roughness, particularly to satisfy surface condition specifications associated with structures used in the application fields mentioned above; and
guarantee good quality in term of defects (surface defects, defects passing through the upper layer of the SeOI structure, etc.), and also to satisfy specifications for the application fields mentioned above.
This type of treatment of the structure obtained directly after detachment typically consists of a sequence of finishing operations. This sequence judiciously leads to one or several steps aimed at thinning the structure obtained after detachment, with one or several steps aimed at smoothing the free surface of the structure.
Thinning part of the donor substrate transferred onto the support substrate after detachment enables one to achieve the target thickness, and this typically is implemented by one or several sacrificial oxidation and/or polishing operations. A polishing operation is usually not desirable because such an operation reduces the uniformity of the thickness of the transferred layer. Thus, when a polishing operation is built into the finishing sequence, it is impossible to guarantee that the standard deviation of the thickness of the transferred layer will be less than 5 Å over the entire thin layer.
Application of a sacrificial oxidation operation is also limited. Such operations tend to cause some pre-existing defects to pass through and into the thin layer, leading to the formation of HF type defects, particularly when a large part of the thickness of the transferred layer is eliminated or removed. In order to limit the formation of such defects, an intermediate thermal annealing operation is usually made between two sacrificial oxidation steps, each of these two steps being designed only for a limited thinning of such layers. As an example of such a thermal annealing, a fast thermal annealing is typically done under a controlled atmosphere using a mode commonly called RTA (Rapid Thermal Annealing). This has been implemented to various degrees in the prior art.
PCT application WO 03/009366 discloses a finishing treatment step that is based on a “basic” sequence (that could be repeated) including a sequence consisting of a sacrificial oxidation operation followed by an RTA operation. It has been observed that a single RTA operation is inefficient, particularly in terms of reducing the surface roughness, when it is implemented on the surface obtained directly after detachment.
PCT application WO 2005/013318 suggests to combine a step to create a weakened zone formed by co-implantation with a finishing treatment step based on a “basic” sequence (that may possibly be repeated) including an RTA operation followed by a sacrificial oxidation operation.
Copending application Ser. No. 11/328,061 filed Jan. 10, 2006, the entire content of which is expressly incorporated herein by reference thereto, explains that it is preferable to transfer a thick layer and then thin this thick layer, rather than attempt to transfer a thin layer directly without thinning, if the resulting thin layer is to have a thickness that is less than 1000 Angstroms, and particularly less than 500 Angstroms.
It follows from the above that a finishing treatment for a thick layer requires several sacrificial oxidation operations (each designed for limited thinning only), with RTA smoothing operations being conducted between the sacrificial oxidation operations. It will easily be understood that the use of a large number of finishing operations has an adverse effect on the global cost of the process.
Furthermore, the formation of a structure with a specified final thickness leads to additional calibration of equipment that depends on the residual thickness of the layer to be treated. For example, calibration of the lamps of an RTA type furnace (necessary to guarantee uniform heating of a wafer) is calibrated as a function of the thickness of the upper layer of an SeOI structure. Thus, if a finishing treatment is necessary for a transferred layer with a thickness of the order of 2000 Angstroms immediately after detachment, it may be necessary to perform several RTA smoothing operations between several thinning operations, to obtain a final thin layer with a thickness of 500 Angstroms. It may then be necessary to adapt each RTA smoothing operation to each thickness (initial, intermediate, final) being treated. Such an adaptation can increase the complexity of a finishing treatment with multiple operations.
Therefore, an attempt is made to simplify the finishing operation as much as possible, without compromising the quality of the final structures. In the context of forming a thin layer structure, the finishing operation sequence selected as being preferred consists of repeating the sequence of a rapid thermal annealing operation followed by a sacrificial oxidation operation. This preferred sequence, also called RTA/Sacrox/RTA/Sacrox (where “Sacrox” denotes a sacrificial oxidation), has the advantage of being high performance in terms of quality, but has the disadvantage of including a large number of operations. Thus, there is a need in the art for simplified or improved processes to enhance the processing efficiency of such materials.